Generally, in a plasma processing apparatus, plasma of a processing gas is generated within a decompression processing vessel. Further, a thin film is formed on a processing target object within the processing vessel by a gas phase reaction or a surface reaction of radicals or ions included in the generated plasma, or micro-processing such as etching of a material or a thin film on a surface of the processing target object is performed.
For example, a capacitively coupled plasma processing apparatus includes an upper electrode and a lower electrode arranged in parallel to each other within a processing vessel. A processing target object (e.g., a semiconductor wafer, a glass substrate, etc) is mounted on the lower electrode, and a high frequency power having a frequency (typically, 13.56 MHz or higher) suitable for plasma generation is applied to the upper electrode or the lower electrode. Electrons are accelerated in a high frequency electric field generated between the two facing electrodes by applying the high frequency power, and plasma is generated as a result of ionization by collision between the electrons and a processing gas. A plasma process such as film forming process or etching process is performed under this plasma.
Recently, as a design rule is getting more miniaturized in a manufacturing process of a semiconductor device or the like, higher level of dimensional accuracy is required in, especially, the plasma etching. Further, it is required to increase etching selectivity against a mask or an underlying film and to improve etching uniformity in the entire surface of a substrate. For this reason, pressure and ion energy in a processing region within a chamber tends to be reduced, so that a high frequency power having a high frequency equal to or higher than 40 MHz is used.
However, as the pressure and the ion energy are reduced, an influence of a charging damage, which has been negligible conventionally, can be no more neglected. That is, in a conventional plasma processing apparatus having the high ion energy, no serious problem may occur even when a plasma potential is non-uniform in the entire surface of the substrate. However, if the ion energy is lowered at a lower pressure, the non-uniformity of the plasma potential in the entire surface of the substrate may easily cause the charging damage on a gate oxide film.
In this regard, to solve the above-mentioned problem, a method of pulse-modulating a high frequency power for plasma generation with an on/off (or H level/L level) pulse having a controllable duty ratio (hereinafter, referred to as “first power modulation method”) has been considered effective. According to this first power modulation method, a plasma generation state in which plasma of a processing gas is being generated and a plasma non-generation state in which the plasma is not being generated are alternately repeated at a preset cycle during a plasma etching process. Accordingly, as compared to a typical plasma process in which plasma is continuously generated from the beginning of the process to the end thereof, a time period during which plasma is continuously generated may be shortened. As a result, the amount of electric charges introduced into a processing target object from the plasma at one time or the amount of electric charges accumulated on the surface of the processing target object may be reduced, so that the charging damage is suppressed from being generated. Therefore, a stable plasma process can be performed and reliability of the plasma process can be improved.
Further, conventionally, in the plasma processing apparatus, a RF bias method is widely employed. In this RF bias method, a high frequency power having a relatively low frequency (typically, 13.56 MHz or lower) is applied to the lower electrode on which the processing target object is mounted, and ions in plasma are accelerated and attracted to the processing target object by a negative bias voltage or a sheath voltage generated on the lower electrode. In this way, by accelerating the ions in the plasma and bringing them into collision with the surface of the processing target object, a surface reaction, an anisotropic etching or a film modification may be facilitated.
However, when performing the etching process to form via holes or contact holes by using the plasma etching apparatus, a so-called micro-loading effect may occur. That is, an etching rate may differ depending on the hole size (or dense/sparse pattern), so that it is difficult to control an etching depth. Especially, the etching rate tends to be higher at a large area such as a guide ring (GR), whereas the etching rate tends to be lower at a small via into which CF-based radicals are difficult to be introduced.
In this regard, to solve the above-stated problem, a method of pulse-modulating a high frequency power for ion attraction with a first level/second level (or on/off) pulse having a controllable duty ratio (hereinafter, referred to as “second power modulation method”) has been considered effective. According to the second power modulation method, a period during which the high frequency power for ion attraction is maintain at a relatively high value having the first level (H level) suitable for etching a preset film on the processing target object and a period during which the high frequency power for ion attraction is maintain at a relatively low value having the second level (L level) suitable for depositing polymer on the preset film on the processing target object are alternately repeated at a certain cycle. Accordingly, at an area having a larger (wider) hole size, a proper polymer layer may be deposited on the preset film at a higher deposition rate, so that the etching may be suppressed. Thus, an undesirable micro-loading effect may be reduced, and it may be possible to perform an etching process with a higher selectivity and a higher etching rate.
Patent Document 1: Japanese Patent Laid-open Publication No. 2000-071292
Patent Document 2: Japanese Patent Laid-open Publication No. 2012-009544
Patent Document 3: Japanese Patent Laid-open Publication No. 2013-033856
Conventionally, in the plasma processing apparatus, the aforementioned first power modulation method and the second power modulation method are used in combination or used concurrently. In such cases, modulation pulses having the same frequency are used in high frequency power supplies for these two respective power modulation methods (for example, Patent Document 1, 2, and 3).
However, the present inventors et. al. has investigated an expected effect of each of the first and second power modulation methods while varying a frequency of the modulation pulse as a parameter, and has found out that a pulse frequency with which the characteristic of the first power modulation method is maximally exerted and a pulse frequency with which the characteristic of the second power modulation method is maximally exerted tend to be greatly different. For example, it has been found out that, in a certain process, a pulse frequency with which the effect of suppressing the charge-up damage by the first power modulation method is maximally exerted is around 90 kHz, whereas a pulse frequency with which the effect of reducing the micro-loading effect by the second power modulation method is maximally exerted is around 10 kHz. Accordingly, when using the first and second power modulation methods at the same time, it may be desirable to use modulation pulses having different frequencies that enable to achieve the maximum characteristics of the two kinds of power modulation methods individually in a certain process.
If, however, the pulse frequencies are different when using the first and second power modulation methods at the same time, matching operations of matching devices on high frequency power supply lines that transfer high frequencies from individual high frequency power supplies to the plasma within the processing vessel may become difficult. Especially, a matching device for a first high frequency power pulse-modulated with a pulse having a higher frequency may be affected by a periodic variation in plasma impedance which is in synchronization with a pulse of a lower frequency (generated by on/off of a second high frequency power). As a result, the matching operation of the matching device may become unstable.